Vertically-aligned carbon nanotube membrane with zeolite imidazolate frameworks as selective layer for hydrogen separation
Vertically-aligned carbon nanotube (VACNT) membranes show very high permeation fluxes due to the inherent smooth and frictionless nature of the interior of the nanotubes, however the hydrogen selectivity is low, in the Knudsen range. In this study, vertically-aligned carbon nanotube membranes have been fabricated via gas phase catalytic chemical vapour deposition (CVD) process followed by encapsulation with epoxy. Then the molecularly sieved zeolite imidazolate frameworks (ZIF), were grown on the VACNT membranes via secondary seeded growth, as the gas selective layer. The ZIF-8 layer is well connected with the VACNT membrane surface with a thickness of 5~6 Ám. Our results show that the ZIF-8 layer can significantly improve the selectivity of carbon nnaotube membrane. The ideal H2 selectivities to Ar, O2, N2, and CH4 of VACNT are only 4.46, 4.01, 3.75 and 2.87, respectively; and these values become 7.0, 13.6, 15.1 and 9.8, respectively over ZIF-8 modified VACNT membrane. Therefore, tailoring metal-organic frameworks on the VACNT membrane can be an effective route to improve the gas separation performance of carbon nanotube membrane.